(1) Field of the invention
The present invention relates to a piston designed for use with a magnetic force pickup for contactlessly sensing a position of the piston, including a piston body with a circumferential outer face and a circumferential groove, a sealing washer having two radially inwardly extending flanges, and a permanent magnet which is placed in the washer towards the outer face of the piston and in said groove, the magnet furthermore having generally axial end faces next to and covered by the flanges.
(2) Description of the prior art
Such pistons with their permanent magnets are used together with a pickup or sensor placed in or outside the cylinder in which the piston is placed. The pickup is responsible for producing an electrical signal when the piston in the cylinder has come into a given position, valves then being worked for the input of working fluid into the working spaces of the cylinder so that the direction of motion of the piston is changed.
In the prior art, the piston washer and the permanent magnet have generally been produced as separate parts which have to be separately handled and put in position, this making the system complex. Furthermore in German Gebrauchsmuster patent No. 7,722,958 a suggestion has been made for placing a permanent magnet in the form of a two-part ring in a circumferential groove of the piston (in the form of a single-piece ring) and on the other hand, by moving it outwards, into an inner groove in the washer or gland fixed to the outer face of the piston, so that the permanent magnet is shut in between the piston and the gland. However, this prior art system has shortcomings, inasmuch as it is not completely fluid-tight on the magnet side and furthermore there is a danger of wear because of the washer's not being fully backed up at its sides, this causing a fretting or "kneading" effect as the washer is moved along backwards and forwards on its piston.